The present invention pertains to corn fiber. More particularly, the present invention pertains to methods of extracting monosaccharides from corn fiber. In a further aspect, the invention pertains to methods of separating the monosaccharides obtained from corn fiber wherein the monosaccharides are obtained according to the methods herein.
In the future, it will become increasingly important to develop consumer products from renewable resources, especially from annually renewable resources. Corn is one example of an annually renewable resource that serves as a source of valuable consumer products. Products derived from corn serve an important role in providing useful foodstuffs to the public. Corn provides important products, such as high fructose corn syrup, ethanol, grain and corn oil. While a large percentage of the total portion of corn is utilized to manufacture these substances, as well as other high value products, a significant fraction of corn is utilized for relatively low value products, such as animal feed. Technology that would allow a higher value utilization of the remaining fractions of corn would provide increased value overall from the entire useable portions of corn.
Corn fiber is one under-utilized fraction of corn. Corn fiber is obtained as a major by-product of the milling of corn. Corn fiber comprises the outer hull portion of the corn kernel. It has been estimated that approximately 7 to 10 billion pounds of corn fiber are produced annually in the United States. The fiber is produced at milling facilities and is collected as a relatively homogeneous fraction.
A major source of corn fiber is the wet milling of corn. During this process, the higher value products are removed from corn, such as the germ of the kernel. After extraction of the high value products, the remainder, which generally constitutes corn fiber, is mixed with steep liquor, also a by-product of corn milling. The mixture of fiber and liquor is then normally dried, pelletized and sold as gluten.
Another source of corn fiber is dry milling; corn fiber obtained from dry milling is often referred to as xe2x80x9ccorn bran.xe2x80x9d The bran by-product of dry milled corn fiber, composed primarily of hull, is mixed with other corn by-products and sold as hominy.
Both gluten feed and hominy feed are fairly low-value products. Nonetheless, these products have generally been the only commercial products prepared from corn fiber. Given the low margins of such products made from corn fiber, it is not uncommon for corn fiber to be disposed of outright, instead of undertaking the effort to prepare such low-value products.
Corn fiber makes up a significant (5 to 10 wt. %) portion of the total weight of the corn kernel. Corn fiber itself is made up of a number of components most of which, if extracted, can be commercially valuable. Specifically, corn fiber consists primarily of residual starch (10 to 25 wt. %), hemicellulose (40 to 50 wt. %), cellulose (15 to 25 wt. %), phenolic acids (3 to 5%), with the remainder present as proteins and oils. (See Wolf, et al. Cereal Chemistry, 30(1953), pp. 195-203; Chanliaud, et al., J. Cereal Science, 21(1995), pp. 195-203.) The variations in the reported composition are believed to be due to corn plant variety and growth conditions, as well as the specific methods utilized to isolate the corn fiber.
Hemicellulose is a component of corn fiber that has been of interest commercially. A number of references disclose the extraction of hemicellulose from corn fiber. However, most previous attempts to obtain useful products from corn fiber have focused almost entirely on methods to extract hemicellulose from corn fiber and the properties, particularly the color, of the hemicellulose obtained. These attempts to extract hemicellulose were likely initiated by the fact that hemicellulose has several valuable properties that make it attractive for a number of applications. In a non-exclusive list, some uses for hemicellulose include non-toxic adhesives, thickeners, emulsifiers, stabilizers, film formers and paper additives. (See e.g., Whistler, Industrial Gums, 3d Ed., Academic Press, 1993, pp. 295-308; U.S. Pat. No. 2,772,981; Wolf, et al. Cereal Chemistry, 30(1953), pp. 451-470.)
As indicated by these, as well as other references, hemicellulose can be quite difficult to extract from corn fiber. Because corn fiber hemicellulose is soluble in H2O, it would be expected that hemicellulose would be fairly easy to extract from corn fiber utilizing water or some other non-aggressive solvent. This is not the case, however. Hydrogen bonding and physical entanglement of the hemicellulose with the corn fiber matrix are believed to be in part responsible for the difficulty in extraction. Other reasons for the difficulty in extractability may be due to cross-linking of the hemicellulose to other components of the corn fiber cell wall via covalent bonds between esterified phenolic acid residues and arabinose residues. Protein-polysaccharide linkages may also affect the ability to extract hemicellulose from corn fiber.
Most previous attempts to extract hemicellulose from corn fiber have focused on the use of strongly alkaline materials. This is not surprising, as one definition of hemicellulose is the portion of plants that is extractable by hot alkali treatment.
Various references disclose techniques to extract hemicellulose. For example, U.S. Pat. No. 2,709,699 discloses extraction of corn fiber with aqueous alkali at a pH of from 9 to 13 at from 90 to 115xc2x0 C. for a time sufficient to solubilize hemicellulose so that it can be extracted. In this reference, the hemicellulose was isolated by adjusting the solution pH with an inorganic acid, followed by precipitation of the hemicellulose in ethanol, filtering to remove the hemicellulose and drying.
In another reference disclosing the extraction of hemicellulose from corn fiber, U.S. Pat. No. 4,038,481, corn fiber is treated with alkali to solubilize the hemicellulose. The hemicellulose is then precipitated with a water miscible organic solvent. The solvents utilized are acetone, methanol, ethanol, propanol, isopropanol, isobutyl alcohol, tert-butyl alcohol, or a mixture thereof. There is no disclosure of precipitation with acetic acid in this reference.
A recent reference, WO98/40413, discloses extraction of hemicellulose by heating corn fiber with alkaline hydrogen peroxide; the peroxide may be added at the same time or after an alkaline material, such as NaOH or Ca(OH)2, is added. Significantly, WO98/40413 discloses the hemicellulose extractant as being heated in the presence of the alkaline hydrogen peroxide in order to obtain a suitably white hemicellulose powder from the precipitation step. However, this method is exceedingly dangerous to practice on an industrial scale because of excessive emissions of gas which may lead to significant foaming of the strongly alkaline materials and possibly to explosions.
Furthermore, although hemicellulose itself is a valuable product, the sub-components of hemicellulose are of even higher value. No reference has been located which addresses the extraction of these valuable sub-components from corn fiber. With the invention herein, it has been found that hemicellulose obtained from corn fiber may be subjected to further processing to provide carbohydrate fractions of very high value. That is, in accordance with the invention herein, it has been found possible to extract a number of valuable monosaccharide materials from corn fiber. Also in accordance with the invention herein, it has been found that hemicellulose from corn fiber may be derivatized to form corn fiber arabinoxylan esters and ethers. Methods of processing corn fiber hemicellulose in such a manner are not believed to be disclosed in the prior art.
Other than to obtain hemicellulose, there have been few attempts to exploit the remaining components of corn fiber. A notable recent exception relates to corn fiber oil. Corn fiber oil contains a significant portion of plant sterol esters. These materials have been reported to be useful as nutraceuticals, particularly as hypocholesterolemics. At this time, rice bran oil and tall oil are the major source of plant sterol esters utilized for commercial purposes.
Rice bran has been reported to contain approximately 18 wt. % extractable oil. Of this amount, 0.1 to about 0.8 wt. % comprises a ferulate ester, meaning that rice bran, at most, contains only about 0.08 wt. % ferulate ester. Moreover, the phytosterol esters in rice bran oil are primarily gamma-oryzanols, which are believed to be less effective as hypocholesterolemics.
In contrast, corn fiber oil has been shown to contain approximately 0.54 to 3.5 wt. % extractable oil and, of this, about 6.75 wt. % is a ferulate ester. Corn fiber therefore comprises about 0.12 wt. % ferulate ester, a significantly higher amount of ferulate ester than is present in the most commercially utilized source of hypocholesterolemic oils which are obtained from rice bran.
A recent patent, U.S. Pat. No. 5,843,499, discloses the extraction of corn fiber oil from finely ground corn fiber by utilizing either hexane or supercritical CO2 as a solvent, with hexane being preferred. In this reference, the degree of grinding was demonstrated to be critical in determining the amount of oil obtained from the corn fiber, with a finer grinding of the corn fiber resulting in a greater amount of oil extracted. Drying of the corn fiber was also found to be highly significant to the invention, presumably because when the corn fiber is wet, the hexane extractant will not adequately penetrate the fiber so as to allow satisfactory extraction. However, because a drying step is expensive and time consuming on an industrial scale, it would be highly beneficial to be able to extract phytosterol esters from corn fiber directly without the need for an additional drying step.
As noted, cellulose forms a significant portion of corn fiber. However, cellulose has not been isolated from corn fiber in a form suitable for derivatization into higher value products, such as cellulose esters and cellulose ethers. This is not surprising because the prior art indicates that high purity cellulose was not obtained from the previously utilized methods. For example, U.S. Pat. No. 4,038,481, discussed previously, discloses that the cellulose obtained according to the methods therein contained about 35 wt. % contaminates which were believed to be present in the form of insoluble hemicellulose. This contamination would make it difficult, if not impossible, to utilize the cellulose obtained according to the methods of the ""481 patent for preparation of cellulose derivatives.
In summary, no known reference addresses methods to obtain maximum utilization of the various components of corn fiber. Instead, the references have focused specifically on the optimization of hemicellulose color, and, separately, on methods to extract oil from corn fiber. While these are valuable objectives in and of themselves, in order to make the use of corn fiber an economically viable process, it is necessary to utilize as many components of corn fiber as possible. Furthermore, it is necessary to develop methods to separate each of these valuable components individually while leaving the remainder of the corn fiber so that the further components can be efficiently extracted in order to maximize value.
In one aspect, the invention provides a method of obtaining at least one monosaccharide from corn fiber the method comprises the steps of: (a) heating an aqueous mixture of corn fiber and a liquid; (b) contacting the mixture of step (a) with a protease enzyme, thereby providing a proteolyzed corn fiber and a liquid; (c) separating the liquid from the proteolyzed corn fiber; (d) contacting the proteolyzed corn fiber at least once with an alkaline extractant, thereby providing an insoluble cellulose material and a liquid comprising arabinoxylan; (e) separating the insoluble cellulose material from the liquid comprising arabinoxylan; (f) reducing the volume of the liquid comprising arabinoxylan by removing excess alkaline extractant, thereby providing a concentrated liquid comprising from about 10 to about 50% solids, wherein the solids comprise arabinoxylan; and (g) hydrolyzing the arabinoxylan from step (f) in the presence of a catalyst and a solvent, thereby providing a mixture comprising at least one monosaccharide.
Still further, the invention provides a method of obtaining at least one monosaccharide from corn fiber wherein the method comprises the steps of: (a) heating an aqueous mixture of corn fiber and a liquid; (b) contacting the mixture of step (a) with a protease enzyme, thereby providing a proteolyzed corn fiber and a liquid; (c) separating the liquid from the proteolyzed corn fiber; (d) contacting the proteolyzed corn fiber at least once with an alkaline extractant, thereby providing an insoluble cellulose material and a liquid comprising arabinoxylan; (e) separating the insoluble cellulose material from the liquid comprising arabinoxylan; (f) reducing the volume of the liquid comprising arabinoxylan by removing excess alkaline extractant, thereby providing a concentrated liquid comprising from about 10 to about 50% solids, wherein the solids comprise arabinoxylan; (g) hydrolyzing the arabinoxylan from step (f) in the presence of a catalyst and a solvent, thereby providing a mixture comprising at least one monosaccharide; and (h) separating the at least one monosaccharide from the mixture of step (g).
In yet a further embodiment, the invention provides a method of obtaining at least one monosaccharide from corn fiber wherein the method comprises the steps of: (a) heating an aqueous mixture of corn fiber and a liquid; (b) contacting the mixture of step (a) with a protease enzyme, thereby providing a proteolyzed corn fiber and a liquid; (c) separating the liquid from the proteolyzed corn fiber; (d) contacting the proteolyzed corn fiber at least once with an alkaline extractant, thereby providing an insoluble cellulose material and a liquid comprising arabinoxylan; (e) separating the insoluble cellulose material from the liquid comprising arabinoxylan; (f) reducing the volume of the liquid comprising arabinoxylan by removing excess alkaline extractant, thereby providing a concentrated liquid comprising from about 10 to about 50% solids, wherein the solids comprise arabinoxylan; (g) hydrolyzing the arabinoxylan from step (f) in the presence of a catalyst and a solvent, thereby providing a mixture comprising at least one monosaccharide; and (h) separating the at least one monosaccharide from the mixture of step (g).
Additional advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.